In-guide control of optical propagation

ABSTRACT

We propose a dynamically tunable electro-optic cladding using our proprietary electro-optic material, applied to various circular and planar wave-guides along with specific electrode configuration and excitation electric field format appropriate to that material. Based on the evanescent field coupling phenomena, the proposed device may be used in variable optical attenuators, tunable filters and couplers, etc. Different design of applied electrodes and optical properties of controllable refractive index materials allow polarization dependent or independent, as well as direct or inverse operation regimes of proposed devices.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application based on, and claims benefit under 35U.S.C.§119(e) of, U.S. Patent Application No. 60/334,118.

MICROFICHE APPENDIX

[0002] Not Applicable.

TECHNICAL FIELD

[0003] The present invention relates to controllable optical devices,and in particular to controllable optical devices for controlling thepropagation of light in an optical waveguide.

BACKGROUND OF THE INVENTION

[0004] As is well known in the art, the successful implementation ofoptical communications systems requires devices capable of reliablycontrolling the propagation of light. Basic optical processing functionsrequired in optical communications include, but are not limited to:optical modulation (which includes amplification and attenuation ofoptical power); phase control (delay); and switching. In WavelengthDivision Multiplexed (WDM) and Dense Wavelength Division Multiplexed(DWDM) communications systems, significant channel power imbalance maybe generated and channel equalization become necessary where the abovenoted basic functions must be performed on a per-channel basis.

[0005] In modern high performance optical communications systems, datarates of 10 GHz or more can be encountered on each channel. In addition,using known optical amplification techniques such as Raman pumping andErbium Doped Fiber Amplification (EDFA), optical transmission spans of1000 Km or more can readily be achieved.

[0006] In general, there are two broad classes of known opticalprocessing devices; namely Out-of-Fiber and In-Fiber. Out-of-Fiberdevices typically involve extracting the light out of the transmissionfiber and into a system of micro-optics. Within the micro-opticalsystem, passive elements such as mirrors and lenses are combined withactive elements such as liquid crystal arrays and/orMicro-Electro-Mechanical Systems (MEMS) to perform a wide range ofsophisticated optical control functions. Light emerging from themicro-optical system is then coupled back into the transmission fiber tocontinue toward its destination.

[0007] Out-of-Fiber systems suffer numerous disadvantages, includingdifficulties manufacturing the micro-optical system components,stability of the system during service, and the high optical lossesimposed by such systems. Losses are encountered within the micro-opticalsystem, and more than 1 dB total loss will be encountered when couplingthe light back into the transmission fiber. As a result, Out-of-Fibersystems typically require more optical amplifiers, which furtherincreases the cost of the system.

[0008] In principle, many of the difficulties that are associated withOut-of-Fiber systems could be overcome by In-Fiber systems, in whichlight propagating within the fiber is controlled without removing thelight from the fiber. One of the earliest In-Fiber systems involvedoptical amplification, in which stimulated Brillouin scattering is usedto amplify optical signals propagating within a fiber. Other knowndevices utilize the reflective and transmissive properties of fiberBragg gratings (FBG) and long period gratings (LPG), respectively, inthe UV radiation sensitive core of fibers. See, for example, U.S. Pat.No. 4,474,427 (Hill et al.; U.S. Pat. No. 5,912,999 (Brennan et al.).

[0009] FBGs are spectral filters, which typically reflect light over anarrow wavelength range and transmit all other wavelengths. They alsocan be designed to have more complex spectral responses. Such filterswere widely used in sensing and optical communications, such as add/dropfilters, dispersion compensators, spectrum equalizers, etc.

[0010] Initially, FBG and LPG based devices were fabricated mainly forstatic operation. However, the performance of many optical components isfrequently affected by environmental conditions and dynamic networkconfiguration changes, and thus is strongly time varying. This requiresthe design and fabrication of dynamically controllable devices,especially wavelength selective components, such as tunable filters andvariable attenuators. Some of the presently known solutions use FBGs andLPGs. For example, different ways have been proposed to dynamicallychange the resonant operation conditions of FBGs. These methods arebased on the change of the effective refractive index of the coren_(eff)^(core)

[0011] or the geometrical period Λ_(FBG) of the grating (the resonantBragg reflection wavelength being λ_(R) = 2n_(eff)^(core)Λ_(FGB)

[0012] ). The known methods use thermo-optic, piezoelectric,acousto-optic effects or fiber stressing mechanisms.

[0013] For example, U.S. Pat. No. 5,007,705 (Morey et al.) teaches atunable FBG in which a heating electrode is used to change thegeometrical period Λ_(FBG) of the grating or the effective refractiveindex n_(eff)^(core)

[0014] of the core material. U.S. Pat. No. 5,699,468 (Farries et al)teaches a FBG based variable optical attenuator (filter) in which apiezo-electric transducer is coupled to each FBG. When a transducer isenergized, it compresses or expands the respective grating, therebychanging the grating period and thus its reflection wavelength. Both ofthese devices are highly temperature sensitive and power consuming, bothof which are undesirable, particularly in compact integrated geometries.

[0015] U.S. Pat. Nos. 5,966,493 and 6,370,312 (both to Wagoner et al.)teach tunable optical attenuators in which the cladding of an opticalfiber is side-polished to expose a surface though which lightpropagating in the fiber core can escape. A controllable refractiveindex material is positioned against this surface. Changes in theeffective refractive index of the controllable index material can beused to control the amount of light coupled out of the fiber core U.S.Pat. No. 6,011,881 (Moslehi et al.) teaches a tunable optical filter inwhich the cladding of an optical fiber is side-polished in the vicinityof a FBG. A controllable refractive index material is positioned againstthis surface. Changes in the refractive index of the controllablematerial can be used to vary the refractive index n_(eff)^(core)

[0016] of the core material, and thus the reflective wavelength of theFBG. Because these devices require a side-polished fiber, the opticalproperties of which are also highly dependent on the radius of curvatureand the distance between the polished surface and the fiber core, theytend to be difficult to manufacture. They also tend to be highlysensitive to temperature. Finally, because of the curvature andasymmetry of the side-polished fiber, its optical performance may bepolarization dependent, which in many cases is undesirable.

[0017] There is a different situation (as compared to FBG) for LPG basedtunable devices. In the case of LPG devices, light may be out-coupledfrom the fiber core and propagate in the cladding of the fiber, theproperties of which are easier to change using an external controllablematerial. The resonant condition of light coupling between the core andthe cladding is a function of the difference of the effective refractiveindex of the core mode n_(eff)^(core)

[0018] and the effective refractive indices of the cladding modesn_(eff)^(clad),

[0019] and also of the period of the LPG Λ_(LPG):λ_(R) = (n_(eff)^(core) − n_(eff)^(clad))Λ_(LPG)

[0020] Thus U.S. Pat. No. 6,058,226 (Starodubov) teaches an opticalsystem for selectively filtering and modulating light extracted from thecore to the cladding area by means of a LPG. This energy transfer isproduced by a resonant (and thus wavelength-selective) mode couplingprocess between the co-propagating fundamental core and higher ordercladding modes of the fiber. Different gratings and electrically drivenelements can be combined to provide various types of filters, sensors,modulators and delay lines. The basic element of these differentconfigurations is an electrically sensitive material which is disposedsurrounding the cladding. The application (across the electricallysensitive material) of a voltage causes the refractive index of thematerial to change. This causes a change of the effective refractiveindex of the cladding n_(eff)^(clad),

[0021] which in turn influences the propagation characteristics ofcladding modes. This in turn changes the resonant coupling conditionbetween the cladding and fundamental core modes. Finally, as thisprocess is wavelength sensitive, it changes the spectral transmittancecharacteristics of this LPG based device.

[0022] However, in contrast to FBG-based devices, the period Λ_(LPG) andspectral bandwidth Δλ_(LPG) of LPGs are typically very large (up to 1000times larger than for narrowband FBG) and are not well adapted fornarrow band (e.g., single WDM channel) applications. Note thatStarodubov teaches a device in which a passive (non-tunable) FBG is usedin combination with two tunable LPGs. Thus the electrical modulation(tuning) of the signal frequency still remains quasi broadband, sincethe radiation is extracted from the core by a quasi-broadband tunableLPG filter. In addition, several transfers are performed here betweenthe core and the co-propagating cladding modes. Finally, Starodubov useselectrodes deposited directly on the cladding of the fiber sinceotherwise, the electrodes on opposed sides of the fiber would be too farapart (more than 125 μm), thereby requiring thus much higher voltages toinduce refractive index changes. However, this solution also is verylimited since it inevitably introduces losses of light because of thecomplex refractive index of the material forming the electrode. Theseconditions limit the application of that device as an efficient, lowloss and narrowband-tuning element. At the same time, as mentionedabove, the tunable FBG based filters could be useful for suchapplications. However, the thermal and mechanical methods, which affectthe core zone of the fiber, are not efficient.

[0023] Accordingly, efficient In-Fiber optical devices remain highlydesirable.

SUMMARY OF THE INVENTION

[0024] An object of the present invention is to provide a highlyefficient optical device of In-Fiber control of optical propagation.

[0025] Accordingly, an aspect of the present invention provides anoptical device for controlling propagation of light within an opticalwaveguide. The waveguide includes a core substantially axi-symmetricallysurrounded by a cladding having a substantially fixed index ofrefraction. The optical device includes a control region in which aradial thickness of the cladding is less than a penetration depth of anevanescent field of light propagating in the waveguide core; avariable-index material surrounding the cladding at least within thetuning region; and a controller. The variable-index material has anindex of refraction that is controllable in response to an appliedstimulus. The controller is designed to controllably apply the stimulusto the variable-index material within the control region.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Further features and advantages of the present invention willbecome apparent from the following detailed description, taken incombination with the appended drawings, in which:

[0027]FIG. 1 shows a schematic illustration of an optical device inaccordance with a first embodiment of the present invention;

[0028]FIGS. 2a and 2 b respectively show a graph illustrating a chemicaletching process and a microphotograph of an etched fiber, usable in thepresent invention;

[0029]FIGS. 3a-3 c schematically illustrate variations of the embodimentof FIG. 1;

[0030]FIGS. 4a-d schematically illustrate different liquid crystalmolecular orientations usable in the present invention;

[0031]FIG. 5 is a schematic illustration showing operation of anembodiment of the present invention implemented as a variable opticalattenuator;

[0032]FIG. 6 is a schematic illustration showing operation of anembodiment of the present invention implemented as a variable phasedelay and tunable filter;

[0033]FIGS. 7a-7 c schematically illustrate operation of an embodimentof the invention utilizing two orthogonal pairs of electrodes positionedsequentially within the control region;

[0034]FIGS. 8a-8 c schematically illustrate operation of an embodimentof the invention utilizing two longitudinally overlapping orthogonalpairs of electrodes positioned within the control region;

[0035]FIG. 9 schematically illustrates an embodiment in which multiplepairs of electrodes are positioned sequentially within the controlregion, with at least some electrode pairs being twist oriented at askew angle relative to the previous pair of electrode;

[0036]FIGS. 10a-10 c schematically illustrate operation of an embodimentof the present invention implemented as a quasi-broad-band filter(channel equalizer);

[0037]FIGS. 11a and 11 b schematically illustrate operation of anembodiment of the present invention implemented as a narrow-band filter(add/drop filter); and

[0038]FIG. 12 illustrates a broad-band in-fiber optical processingsystem incorporating a plurality of optical devices in accordance withthe present invention.

[0039] It will be noted that throughout the appended drawings, likefeatures are identified by like reference numerals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0040] The present invention provides an optical device designed tocontrol propagation of light within the core of an optical waveguide,such as a fiber. Variants of the optical device may be used in opticalcommunication systems as a tunable filter, optical attenuator, delayline etc. FIG. 1 schematically illustrates principle elements of anembodiment of the present invention.

[0041] As shown in FIG. 1, the optical device 2 generally comprises anoptical waveguide 4 having a core 6 surrounded by a cladding 8 having asubstantially fixed index of refraction. A control region 10 is formedin the waveguide 4, in which a radial thickness of the cladding 8 isreduced to less than the penetration depth of an evanescent field oflight propagating in the waveguide core 6. A variable-index material 12is disposed around the cladding 8 at least within the control region 10,and thus is capable of interaction with the evanescent field of light inthe core 6. The variable-index material 12 has an index of refractionthat is controllable in response to an applied stimulus. A controller 14is arranged to controllably apply the stimulus to the variable-indexmaterial 12 within the control region.

[0042] Preferably, axial symmetry of the waveguide 4 is maintainedthroughout the length of the control region 10, so that the evanescentfield of light within the core 6 can interact with the variable-indexmaterial 12 independently of its polarizations state. As will bedescribed in greater detail below, this enables the optical device 2 toprovide polarization-dependent and/or polarization-independent controlfunctionality, as desired.

[0043] In the embodiment of FIG. 1, the core 6 is unmodified. In thiscase, controlling the index of refraction of the variable-index material12 has the effect of controlling the coupling of light out of the core6. This effect usually is not wavelength selective, at least within afew tens of nm, so the embodiment of FIG. 1 provides a broadbandvariable optical attenuator (VOA). If desired, the core can be modifiedby the introduction of transmissive LPGs and/or reflective FBGs, toprovide more sophisticated functionality, as will be described ingreater detail below.

[0044] Various methods may be used to fabricate the control region 10within a premanufactured waveguide, such as, for example, polishing oretching. Alternatively, the control region can be fabricated duringmanufacture of the waveguide cladding 8, in which case other knowntechniques may be more appropriate. In embodiments in which aconventional optical fiber is used as the waveguide 4, etching of thecladding 8 can advantageously be used to reduce the cladding thicknesswhile preserving axial symmetry within the etched region of the fiber.As shown in FIG. 2a, the depth of chemical etching of an optical fiberusing a hydrofluoric (HF) acid solution, is an approximately linearfunction of time. The rate of the chemical reaction depends primarily onthe acid concentration and may be well controlled. Moreover, usingmasks, bent structures and/or selected time schedules, it is possible toconstruct non-uniform, periodic and quasi-periodic etched structureswithin the control region 10 of the waveguide. These structures can beused to generate desired spectral and polarization properties of theoptical device. FIG. 2b shows a microphotograph of the transition zoneof a typical etched fiber.

[0045] As may be appreciated, a variety of different variable-indexmaterials may be utilized. In principle, any suitable thermo-optical,magneto-optical, and/or electro-optical materials that exhibit a changein refractive index in response to an applied stimulus may be used.However, the variable index material is preferably selected to alsoallow low energy consumption, fast response time, and easycontrollability. Thus, for example, thermo-optical materials, in whichthe refractive index varies with temperature, are not preferred becausethey typically require high power consumption, slow response time(particularly for cooling), and thermally-induced dimensional changes inthe waveguide material itself. As the modal behavior (fielddistribution, phase, etc.) within the fiber core is sensitive to therefractive index of the surrounding media, it is important to utilize avariable-index material having a respective refractive index thatpreferentially lies close to the refractive index of the cladding 8. Abent structure of the waveguide 4 may also be used to monitor theevanescent field penetration into the controllable material area.

[0046] Preferably, the variable index material 12 is either a compositeliquid crystal (LC), or an electro-optical polymer. Both of thesematerials exhibit continuously variable index of refraction in responseto an applied electric field. Because they are dielectrics, current flowis minimized (almost to zero) so that resistive heating (and thusundesired thermal effects) is virtually eliminated. Finally, theresponse time for these materials can be very fast, particularly for theelectro-optical polymers. Composite polymer LC materials have advantagesin that they are readily available at low cost, have understood opticalproperties, and their incorporation into practical devices can bereadily accomplished. Electro-optical polymers, such as POLY (DR1-MMA)available from IBM, are advantageous in that they are solids at normaloperating temperatures, which is particularly beneficial in theconstruction of integrated devices.

[0047] In embodiments utilizing electro-optical variable index materials12 (e.g., LCs and electro-optical polymers), the index of refraction canbe continuously controlled by means of an electric field applied to thevariable-index material 12. Accordingly, as shown in FIG. 1, a pair ofelectrodes 16 can be disposed on opposite sides of the core 6, andcoupled to the controller 14. Thus the controller 14 can apply aselected voltage difference to the opposed electrodes 16 a,b to generatea desired electric field, which will be oriented transversely to thecore 6. The separation distance between the electrodes can be less than30 μm, so that required electric field strengths can be generated atcomparatively low voltages.

[0048] Various well known methods may be used to fabricate theelectrodes 16. For example, Indium Tin Oxide (ITO) electrodes can beformed in a glass or silicon substrate (not shown) using techniques wellknown in the art. Other metallic electrodes (e.g., of gold, silver oraluminum) can also be formed using known techniques, such as vapordeposition. Since transparency of the electrodes is often not required,bulk metallic electrodes may also be used.

[0049] In the embodiment of FIG. 1, a single pair of electrodes 16 isshown spanning substantially the entire length of the control region 10.However, if desired, each electrode 16 may be divided into two or moreelectrode elements 18 (FIG. 3c), each of which may be independentlycontrolled. If desired, electrode elements can be aligned with the core6, or may be oriented at an angle to the core 6, as desired. FIGS. 3a-3c illustrate various features that may be incorporated (either alone orin combination) into the optical device, in order to achieve a desiredoptical performance characteristic.

[0050] Thus, FIG. 3a illustrates the embodiment of FIG. 1 with theaddition of a Fiber Bragg Grating (FBG) 20 written into the core 6 usingknown techniques. Such an FBG may take the form of a periodic variationof the refractive index of the core 6, or may be more complex. Theperiodicity of the FBG may be constant, or may vary, as desired.Multiple FBGs may be used within the same control region, if desired,and may be associated with (that is, may lie within the electric fieldgenerated between) a common electrode pair, or respective differentelectrode pairs, as desired. In any case, variations in the refractiveindex of the variable index material surrounding the FBG will have theeffect of changing the reflective wavelength of the FBG. Thus opticaldevices incorporating one or more FBGs will function as tunablenarrow-band attenuators, add-drop filters, etc.

[0051]FIG. 3b illustrates an embodiment in which the process of etchingthe cladding has been controlled to produce a periodic variation of thecladding thickness along the length of the control region 10. Note thataxial symmetry is maintained throughout the control region, even as thecladding thickness varies with longitudinal position.

[0052]FIG. 3c illustrates an embodiment in which multiple electrodeelements 18 are arranged along the length of the control region. Ifdesired, the electrode elements 18 can be independently controlled sothat the index of refraction of the variable index material 12 can varywith longitudinal position within the control region 10.

[0053] As mentioned previously, LC materials are particularly useful forthe present invention. LC materials are known to exhibit highelectro-optic sensitivity and optical birefringence Δn (up to 0.28)between ordinary and extraordinary refractive indices (denoted as n_(o)and n_(e), respectively). The refractive index “seen” by light incidenton a LC material is a function of the angle between the polarizationvector of the incident light and the local averaged direction ofmolecular long axes (commonly called the director). An importantconsideration is the initial alignment of the LC molecules at theinterfaces defined by the cladding surface (within the control region)and the surrounding surfaces; that is, the electrodes 16, theirsupporting substrates and adjoining walls that contain the LC material.Depending on the desired initial (or relaxed-state) alignment, varioustechniques can be used to achieve the required anchoring conditions andLC alignment at the scale of the cell.

[0054]FIG. 4 shows different basic LC alignments at the claddingsurface. FIG. 4a shows a planar homogenous alignment with the directorpointing along the fiber axis. FIG. 4b shows a radial or homeotropicalignment. FIG. 4c shows a planar azimuthal alignment, and FIG. 4d showsa tilted alignment. Hybrid geometries may be obtained when the anchoringat various points or boundaries are different. For example, FIG. 4eillustrates a geometry in which different LC surface alignments areestablished at opposed cell surfaces. In this case, the director fieldcan be non-uniform in space, as shown in FIG. 4e.

[0055] If desired, the director orientation may be constant throughoutthe length of the control region. However, different directororientations can be established along the length of the control regionto obtain desired (e.g., polarization independent) properties of theoptical device, as will be described in greater detail below.

[0056] The most frequently used technique for establishing therelaxed-state (i.e., with no applied electric field) directororientation is the unidirectional rubbing or photoexposition of apolymer film at the solid-LC interface. When applied to the claddingsurface within the control region 10, this produces a uniform planarorientation as shown in FIG. 4a. Another common orientation geometry isthe homogeneous alignment of the LC molecules perpendicular to theinterface (see FIG. 4b). This texture is commonly called homeotropic andis achievable using surfactant materials doped in the LC or deposited onthe cladding surface. Cetyl-Trimethyl-Ammonium-Bromide (CTAB) andlecithin are commonly used as surfactants for this purpose. Generally(with CTAB for example) nearly monomolecular layers are formed, and sothis additional layer does not perturb the optical properties of thedevice. Truly monomolecular layers of molecules having long methylenechains are also a good technique to provide strong homeotropicalignment. These monolayers may be coated on the surface using, forexample, the well known Langmuir-Blodgett or similar depositiontechniques. One can also obtain the described textures by applyingexternal electric or magnetic fields, if desired.

[0057] Another case is dealing with all possible tilted and hybridalignments with respect to the interface plane (see FIGS. 4d and e).Many techniques can be used to obtain tilted polar anchoring, such asoblique evaporation of SiO₂ or MgF₂, relief gratings, pixilatedelectrodes, photo-induced anisotropy, etc.

[0058] For each initial director alignment, there is a correspondingrefractive index profile with respect to the fiber core-modes. Forexample, the planar homogeneous orientation, when the director ispointing along the fiber axis (FIG. 4a) provides an isotropic andconstant refractive index distribution.

[0059] Other techniques, such as the use of pixilated electrode arrays,photosensitive layers, light exposition, particle deposition or chargedbeam exposition, may also be used to achieve a desired directoralignment. For example, an external magnetic field or a surface layercontaining an oriented magnetic powder may be applied to achieve thedesired local surface orientation. It should be noted that the discussedalignment techniques may be usable for both planar and cylindricalgeometry waveguides.

[0060] Well known in the art alignment methods can provide high or low(approaching zero) interaction energy between LC molecules and aninterface (e.g., cladding surface or electrode substrate surfaces) toobtain the desired, polar and azimuthal anchoring. Azimuthal anchoringwith near zero interaction energy is achievable, for example, withfluorinated materials (like Teflon). Consequently, the requiredoperating voltages are lower, and there is almost no threshold. Also,there is almost no significant radial dependence of the refractive indexin the vicinity of the cladding surface. With this arrangement, crossedcouples of electrodes (for example placed along the length of thecontrol region) can be used to construct a rapid LC modulator.Threshold-free operation may be also achieved using pre-tilt angles onorienting surfaces, which in addition would accelerate the response ofthe device. In the particular case of cylindrical interface geometry(i.e., the fiber cladding surface), liquid crystals can naturally orientalong the symmetry axis (longitudinal or z-axis of the waveguide) tominimize distortion energy or, in other words, to avoid bend, splay andsaddle-splay deformations that “cost” more elastic energy than theuniform alignment. This has been experimentally observed by theinventors when no special surface treatments were made on an etchedsilica optical fiber.

[0061] As mentioned above, the general mode of operation of the opticaldevice of the present invention is based on the application of anexternal stimulus such as an electric field, which results in the directchange of the refractive index of the variable-index material andconsequently changes the effective refractive index “seen” by lightpropagating in the core. In the case of LC variable-index materials,this change may be accomplished via reorientation of the initial(relaxed-state) direction of the LC molecules with respect to thewaveguide core. The selection of materials used, electrode geometry,gratings written to the wave guide core, and cladding contour (producedby controlled etching, for example) can be combined in various ways toconstruct a variety of different optical devices. In order to illustratethe versatility of the present invention, a selection of differentdevices are described below. It will be recognized that the describedselection is in no way exhaustive, and therefore is not limitative ofthe scope of the appended claims.

[0062] Variable Optical Attenuator

[0063]FIG. 1 illustrates a basic embodiment of the present invention,which can be used to obtain broadband variable optical attenuation orphase delay. Variable optical attenuation is obtained by selecting andcontrolling the variable-index material such that the maximum andminimum refractive indices n_(MAX)^(EO)

[0064] and n_(MIN)^(EO)

[0065] of the variable index material (as seen by light propagating inthe core) are respectively above and below the effective refractiveindex n_(eff) of the core modes (which is generally slightly below therefractive index n^(core) of the core). Conversely, variable phase delayis obtained by selecting and controlling the variable-index materialsuch that the maximum achievable refractive index n_(MAX)^(EO)

[0066] of the variable index material is equal to or less than theeffective refractive index n_(eff) of the core modes. These modes ofoperation are illustrated in FIGS. 5 and 6, in which n_(clad) is therefractive index of the cladding. Some commercially available opticalfibers have n^(core)=1.47, and various values of n_(clad) (belown^(core)) may be obtained by doping, implanting or deposition. At thesame time, the present inventors have developed some composite polymerLC materials having refractive indexes which may be varied by anelectric field from 1.42 to 1.75. It is therefore readily possible tocontrol such an LC material to obtain the desired mode of operation.When the refractive index n^(EO) of the variable index material is belowthe effective refractive index n_(eff) of the core modes, refractiveindex changes will modify the effective refractive index of the coremodes. Consequently the propagation constant of these modes will bechanged, resulting in a phase delay that will be a function of thedifference between n^(EO) and n_(eff). To the extent that refractiveindex is wavelength-dependent, the phase shift will also vary withwavelength.

[0067] As the refractive index n^(EO) of the variable index materialincreases above n_(eff), light will be coupled out of the core,resulting in attenuation of optical modes within the core. The amount oflight coupled out of the core is a function of the difference betweenn^(EO) and n_(eff), and will vary with wavelength as described above inconnection with variable phase delay. Thus broadband variable opticalattenuation of core modes is achieved. The present inventors haveexperimentally obtained a 55 dB attenuation using an attenuator inaccordance with the present invention. Note that either n_(MAX)^(EO)

[0068] and n_(MIN)^(EO)

[0069] may correspond to the relaxed-state of the LC variable-indexmaterial, so that direct or reverse modulation mode variableattenuations (described below) may be obtained, as desired.

[0070] Tunable Filter

[0071] A tunable reflection of light propagating in the core can beobtained from the device of FIG. 1, by adding a FBG to the waveguidecore, and by choosing the variable-index material such that the maximumrefractive index n_(MAX)^(EO)

[0072] of the variable index material (as seen by light propagating inthe core) is below the effective refractive index n_(eff) of the coremodes. This mode of operation is illustrated in FIG. 6. As mentionedabove, by keeping n_(MAX)^(EO) < n_(eff),

[0073] refractive index changes modify n_(eff), resulting in changes ofthe propagation constant of these modes. This has the effect of varyingthe resonant (reflective) wavelength of the FBG within the core, therebyallowing the FBG to be tuned to a desired wavelength. This will happenwithout changing the geometrical period of the grating and without achange of the refractive index of the core material itself.

[0074] Consider a practical case in which n_(e) and n_(o) arerespectively above and below the refractive index n_(clad) of thecladding, that is, n_(e)>n_(clad)>n_(o). Two key geometries will thenallow us to demonstrate the principles of operation of the proposeddevices. First, consider an LC cell which has surface treatmentsproviding planar molecular orientation with the LC director parallel tothe waveguide longitudinal axis (i.e., the z-axis, see FIG. 4a). In thiscase, both TE and TM fundamental modes within the waveguide core see aneffective refractive index, which is mainly determined by the ordinaryrefractive index n_(o) of the LC material. The application of anexternal field across the waveguide will result in the reorientation ofLC molecules. The new orientation of the director will tend to beparallel with the applied field if the LC has positive dielectric Δε>0(or magnetic Δμ>0) anisotropy at the frequency of the electric (ormagnetic) field used. In this case, one of polarization modes will seedifferent (e.g., higher) effective refractive index of the LC material,which will consequently change its propagation constant. There will beno losses if the achieved refractive index n^(EO) remains smaller thanthe effective refractive index n_(eff)^(core)

[0075] of the core modes. In the case when n^(EO) becomes higher thann_(eff)^(core),

[0076] then we obtain a mode which is at least partially coupled outfrom the core, and thereby suffers controllable losses. At the sametime, this device may have very low insertion loss (less than 0.1 dB)since the light remains within the core of the waveguide before theelectric field is applied (when n_(eff)^(core)

[0077] is higher than n_(o)).

[0078] Note that with only one electro-optic cell (with simple planarelectrodes as shown in FIG. 1, the described modulation (attenuation,reflection or phase shift) is in general polarization dependent, whichcould be used to create electrically tunable polarizers and anisotropicphase delays. Thus a switchable fiber polarizer with 30 dB polarizationextinction ratio has been demonstrated by the present inventors.

[0079] In some cases however, the polarization dependence may be anundesirable effect. It is then possible to make these elementspolarization independent using multiple couples of electrodes that allowthe application of orthogonal electric fields over many portions of thefiber. For example, one can use the electrode configurations shown inFIGS. 7 and 8, in which different electric fields E₁ and E₂ are appliedalong the length of the control zone. E₁ and E₂ may or may not beorthogonal, as desired. In principle, there is no limitation on thenumber and length of electrodes (or electrode elements) that can bepositioned along the control region. Polarization independent operationcan also be realized using LC cells with chiral contents or initialhybrid alignment (see below).

[0080] Thus, an embodiment of the present invention utilizes arelatively long FBG written into the core, with two mutually tilted(e.g., orthogonal) pairs of electrodes, which are sequentiallypositioned within the control zone, as shown in FIG. 7. This designallows each polarization mode of input light to be independentlycontrolled.

[0081] Another variation utilizes the overlapping (longitudinally) oftwo or more pairs of electrodes at respective different angularorientations, as shown in FIG. 8. This electrode geometry allows richerchoices of the spatial form of the applied electric field via theswitching on and off of different couples of electrodes (vertical,horizontal, tilted right, tilted left, etc.), and thus improvedpolarization manipulations.

[0082] The use of multiple overlapping electrode pairs allows theresponse time of the LC molecular reorientation to be significantlydecreased by dynamically switching power between appropriate electrodepairs, as may be seen in FIG. 8. it is also possible to use an LCcomposition which has a dual-frequency operation, that is, positive andnegative dielectric anisotropy Δε at different driving frequencies(e.g., ω₁ and ω₂). Thus, the switch-off of the forward driving signal(at frequency ω₁) and switch-on of the back driving signal (at frequencyω₂) will accelerate the modulation cycle.

[0083] Still another variation of the present invention may includehelicoidal or spatially distributed pixilated and angularly spread(tilted in an optimal manner) individual electrode pairs (as isschematically shown in FIG. 9) arranged to have parallel (to the fiber)surfaces, but mutually twisted around the fiber to create a desiredspatio-temporal distribution of the electric field with correspondingmodulation of the effective refractive index. This would allow thecreation and tuning of polarization properties of the device, such aspolarization insensitive or highly sensitive operation, etc. Note thatthe application of twisted polarization maintaining fibers orhelical-core fibers also may be used to improve the polarizationproperties of optical devices in accordance with the present invention,even where complex electrode configurations are not used.

[0084] Another configuration can use a twisted (along the guide axis)magnetic field, which introduces losses for both TE and TM modes. Thismay be achieved, for example, by means of an externally applied field oran orienting layer which contains periodically oriented magnetic powderentrained within a solid matrix. Then the switch to the transparent modemay be achieved by applying a stronger magnetic or electric field alongthe guide axis or simply removing the previous orienting field.

[0085] The polarization dependence of embodiments utilizing the LCdirector orientation of FIG. 4a, but with positive dielectric anisotropyΔε>0, can also be reduced if two or more mutually crossed and phaseshifted oscillating electric field components are applied across thefiber. In this case, one can achieve an orientational transition fromuniform planar (FIG. 4a) to uniform homeotropic (FIG. 4b) states (andvice versa). The same transition could also be achieved by applying alongitudinal field (along the fiber axis and initial directororientation) if the LC used has negative dielectric (Δε<0) or magneticanisotropy (as described below).

[0086] In another geometry, an initial homeotropic orientation of the LC(FIG. 4b) can be used. In this case, both polarization modes willinitially see the same average refractive index n_(av) of the LC. Theywill suffer from losses if n_(av) is higher than the n_(eff) of the core(n_(eff)^(core))

[0087] or acquire some phase delay if n_(av) < n_(eff)^(core).

[0088] The application of a transverse field will reorient the LCdirector as described above, thereby increasing the refractive index forthe extraordinary mode and decreasing the refractive index for theordinary mode (if n_(e)>n_(o)). These changes will generate leakagelosses as soon as the refractive index n^(EO) is larger thann_(eff)^(core).

[0089] In a completely reoriented state, the fiber would attenuate theextraordinary mode (if  n_(eff)^(core) < n_(e))

[0090] and be transparent for the ordinary mode (if(if  n_(eff)^(core) > n_(o)).

[0091] This polarization dependant operation can be avoided by applyingan electric or magnetic field along the fiber axis (for the LC havingpositive dielectric anisotropy Δε>0). In this case, it is possible toobtain a transition from the state represented in FIG. 4b to the staterepresented in FIG. 4a. Note that in this geometry, the attenuationtakes place when the applied voltage is zero. This reverse-modefunctionality is important for protecting expensive optical devices(e.g., for detection, test and measurements) from optical power damagedue to power supply and attenuator failure.

[0092] The above-mentioned operation modes may be changed using amaterial which has a negative dielectric anisotropy (Δε<0) at theexternal excitation frequency. In this case, application of the electricfield will repulse the director from that field and align it in theperpendicular direction. In this way, some useful combinations can becreated using various initial orientations, electrode forms andgeometries, and LC materials with negative and positive dielectricanisotropy (e.g., at low or quasi-DC and high frequencies) and opticalanisotropy values. For example, the application of two (or more) crossedfields, which are transverse (e.g., along y and x axes) with respect tothe longitudinal axis of the waveguide and which are oscillating at anoptimal frequency and relative phase shift, would force the LC directorto be parallel along the z-axis producing again the same transition fromthat of FIG. 4b to that of FIG. 4a.

[0093] Another way to make the above described devices to bepolarization insensitive is the use of a twisted (along the guide axis)and bent fibers (including polarization maintaining and helical-corefibers), which are placed in the simple electro-optic cell. The use of adepolarizer, placed at the entrance of the cell, can also make theoperation polarization independent.

[0094] As mentioned above, it is possible also to use a hybrid LCalignment as shown in FIG. 4e. In this case (e.g., with Δn>0), whenthere is no electric field E, the attenuation of the vertical (in thefigure) polarization component occurs mainly in the top portion of theLC cell (which has a homeotropic alignment), while the attenuation ofthe horizontal (in the figure) polarization component takes place in thebottom portion of the cell (having a planar alignment). The applicationof electric or magnetic fields along the waveguide axis will result inthe reorientation of LC molecules along the fiber axis (for Δε>0)resulting in a reduction of the leakage losses ifn_(eff)^(core) > n_(o).

[0095] Note that this will affect both polarization modes effapproximately equally. The same switch may also be achieved using two ormore phase-shifted across-fiber oscillating fields for Δε<0. Note thatin this configuration we also have a reverse mode operation, when theattenuation takes place when the applied voltage is zero.

[0096] Optical devices constructed in accordance with the presentinvention should preferably have a fast response time and high dynamicrange (modulation depth). In the present invention, the dynamic rangedepends on the length of the control region and the portion of theevanescent field of the core mode that penetrates into thevariable-index material. The response time of a given electro-optic LCmaterial, which is generally limited by the relaxation time (except thecase of forced relaxation geometries), usually depends on the cellthickness (i.e., the distance between LC boundaries or electrodes). Thedistance between the fiber and the nearest point of an electrode surfaceshould preferably be far enough to prevent the introduction of losses.Explicitly, to decrease the response time of the LC material in anelectric field, it is necessary to have low viscosity γ, smallreorientation scale, etc. Also, the response is faster for higherelectric fields, so it is typically faster for higher attenuationvalues. In the case where low attenuation is needed while keeping a fastresponse, the above-described electrode configurations can be dividedinto multiple discrete electrode elements arranged along the length ofthe control region, and these electrode elements can be drivenindependently (either step-wise or continuously) over a limited range ofrelatively high voltage values. In the case of short and numerouspixilated electrodes, one can reach very low attenuation levels, whichcan be increased as desired by activating additional electrode elements.This multiple electrode configuration also contributes to reducedrelaxation time, since the volume of reoriented LC molecules is smaller.As mentioned above, electrodes can be made in such a way as to createorthogonal fields in order to minimize the polarization dependent losses(PDL). The same configuration can also be used to achieve discrete phasetuning in the case where n^(EO) < n_(eff)^(core).

[0097] Such pixilated electrodes may be positioned with relative(triple) spatial shifts in all x, y and z directions to generateorienting fields in 3D for excitation and also for forced relaxationapplications.

[0098]FIG. 10 illustrates operation of a tunable filter element asdescribed above, deployed as a dynamic gain equalizer. FIG. 10a shows areflective FBG's transmission curve superimposed over an example of thespectrum of four channels, at a position upstream of the tunable filter.FIG. 10b shows the channel spectrum after equalization by the filter. Inthis configuration, the FBG's transmission spectrum can be tuned(shifted) with respect to the channel spectum to obtain different levelsof reflection and attenuation. It is possible also to use a chirped FBG20 (see FIG. 10c) to obtain a comparatively broadband filter.

[0099] Another useful application of the present invention is thenarrowband monitoring (attenuation or reflection, add or drop, etc.) ofindividual channels in WDM systems. In this case the transmissionspectrum of the FBG used should preferably be not broader than thechannel separation (typically on the order of 0.2-0.4 nm) As may be seenin FIG. 11, by means of tuning the resonant wavelength of the FBG (asdescribe above), it is possible to selectively modulate, add or dropselected individual channels. In FIG. 11, the dropped channel'swavelength is the second from the right side, which is resonant for FBGwith corresponding applied voltage.

[0100] The above-described devices may use not only gratings recorded inthe volume of the materials which compose the waveguide or thesubstrate, but also other kinds of gratings created, for example, withinthe cladding (see FIG. 3b) and/or the core of the waveguide. Therefractive index of the variable-index material can also be tuned usingother means such as temperature modulation, mechanical stress, externalirradiation, etc.

[0101] An important application of proposed devices relates to theproblem of multi-channel non-uniform amplified signal flattening orchannel equilibration after add/drop filtering. This application isbased on the combination of the above-described equalizer and/or basicnarrowband devices, for in-line equalization of multi-channel amplifiedsignals. In this scheme, multiple tunable elements are positionedsequentially along a fiber, as shown in.. 12, each being designed toenable tuning across a respective range of wavelengths. With thissequential arrangement, it is possible to manipulate (e.g., equalize,etc.) the signals of individual channels across the full operatingbandwidth of the optical signal traffic, by suitably tuning the relevantdevice.

[0102] Still in another embodiment of the present invention, it ispossible to apply the same approach of evanescent field modulation tovarious kinds of arrayed fibers or integrated optics circuits, couplers,tapered fiber components, multiplexers and demultiplexers, to change theeffective refractive index of the guiding part of these devices and totune their operation conditions. An example of such a use is the tuningof the add/drop filters and the reconfiguring of arrayed waveguides.Thus an integrated arrayed waveguide device may be created, where thetuning of optical paths may be done using controllable material andcorrespondingly patterned electrodes (on the same guiding substrate oron an upper substrate to control each portion of the material separatelyby transverse or longitudinal fields, as described above). This allowsthe reconfiguration of the device via tuning of optical paths (whenn^(EO)<n_(eff)) at each guiding channel separately while the use ofvariable optical attenuators (when n^(EO)>n_(eff)), which are preferablyplaced on already separated WDM channels would allow the monitoring oftransmissions of separate channels. A combined operation (simultaneousphase and amplitude modulation) is also possible, via the crossing ofthe refractive index of the cladding layer by the refractive index ofthe controllable material.

[0103] There are many other applications of the present invention, whichwill be apparent to those skilled in the art.

[0104] Experimental Results

[0105] A Variable Optical Attenuator constructed as described aboveexhibited 55 dB attenuation using a 30V square-wave electric signalhaving a frequency of 1 kHz. The device had low insertion loss (<0.1dB). The recorded rise and decay times were 10 ms and 100 msrespectively. A relatively low PDL of 0.28 dB was obtained using theabove described multiple electrode structure.

[0106] The embodiment(s) of the invention described above is (are)intended to be exemplary only. The scope of the invention is thereforeintended to be limited solely by the scope of the appended claims.

We claim:
 1. An optical device for controlling propagation of lightwithin an optical waveguide comprising a core substantiallyaxi-symmetrically surrounded by a cladding having a substantially fixedindex of refraction, the optical device comprising: a control region ofthe optical waveguide in which a radial thickness of the cladding isless than a penetration depth of an evanescent field of lightpropagating in the waveguide core; a variable-index material surroundingthe cladding at least within the control region, the variable-indexmaterial having an index of refraction that is controllable in responseto an applied stimulus; and a controller adapted to controllably applythe stimulus to the variable-index material within the control region.2. An optical device as claimed in claim 1, wherein, at least within thecontrol region of the optical waveguide, the thickness of the claddingis substantially uniform in a longitudinal direction of the waveguide.3. An optical device as claimed in claim 1, wherein, at least within thecontrol region of the optical waveguide, the thickness of the claddingvaries with longitudinal position.
 4. An optical device as claimed inclaim 1, wherein the variable-index material comprises a liquid crystalmaterial, and the stimulus comprises an electric field applied to thevariable-index material substantially transversely to a longitudinalaxis of the waveguide.
 5. An optical device as claimed in claim 4,further comprising a surface treatment applied to an exterior surface ofthe cladding for defining a predetermined relaxed-state orientation ofthe liquid crystal in the absence of an electric field.
 6. An opticaldevice as claimed in claim 5, wherein the relaxed-state orientationcomprises any one or more of: a parallel orientation in which moleculesof the liquid crystal lie substantially parallel to the longitudinalaxis of the waveguide; a radial orientation in which molecules of theliquid crystal lie substantially radially about the longitudinal axis ofthe waveguide; a circumferential orientation in which molecules of theliquid crystal lie substantially circumferentially about thelongitudinal axis of the waveguide; and a helical orientation, in whichmolecules of the liquid crystal lie in a substantially helical pathabout the longitudinal axis of the waveguide.
 7. An optical device asclaimed in claim 5, wherein the relaxed-state orientation is the samethroughout the control portion.
 8. An optical device as claimed in claim5, wherein the relaxed-state orientation is different at respectivedifferent longitudinal positions within the control portion.
 9. Anoptical device as claimed in claim 4, wherein the controller comprisesat least two electrodes disposed substantially symmetrically about thewaveguide and defining a predetermined energized-state orientation ofthe liquid crystal in accordance with an electric field between theelectrodes, the energized-state orientation of the liquid crystal beingat least partially transverse to the core of the waveguide.
 10. Anoptical device as claimed in claim 9, wherein the at least twoelectrodes comprises a first pair of electrodes disposed on oppositesides of the waveguide core.
 11. An optical device as claimed in claim10, wherein the at least two electrodes comprises a second pair ofelectrodes disposed on opposite sides of the waveguide core, the secondpair of electrodes being controllable independently of the first pair ofelectrodes, and being angularly separated from the first pair ofelectrodes.
 12. An optical device as claimed in claim 9, wherein theelectrodes are oriented substantially parallel to the core.
 13. Anoptical device as claimed in claim 9, wherein the electrodes areoriented at an angle to the core.
 14. An optical device as claimed inclaim 9, wherein a length of the electrodes substantially correspondswith the length of the control portion.
 15. An optical device as claimedin claim 14, wherein each electrode is electrically contiguous, suchthat a substantially uniform electric field is generated within thecontrol portion.
 16. An optical device as claimed in claim 14, whereineach electrode is divided into a plurality of independently controllableelectrode elements, such that a varying electric field can be generatedwithin the control portion.
 17. An optical device as claimed in claim 1,wherein the variable-index material is a solid-state material.
 18. Anoptical device as claimed in claim 17, wherein the solid-state materialis a birefringent material having a principle axis.
 19. An opticaldevice as claimed in claim 18, wherein the birefringent material isdivided into at least one axial segment within the control portion, theprinciple axis of the birefringent material of each segment beingarranged in a selected one of a plurality of orientations relative tothe longitudinal axis of the waveguide.
 20. An optical device as claimedin claim 19, wherein the plurality of orientations comprises: a parallelorientation in which the principle axis lies substantially parallel tothe longitudinal axis; transverse orientation in which the principleaxis lies substantially transverse to the longitudinal axis; angledorientation in which the principle axis lies at an angle relative to thelongitudinal axis; and a helical orientation in which the principle axisfollows a substantially helical path about the longitudinal axis.
 21. Anoptical device as claimed in claim 19, wherein the selected orientationof the principle axis of the birefringent material is the same in eachsegment.
 22. An optical device as claimed in claim 19, wherein theselected orientation of the principle axis of the birefringent materialin one segment is different from that of at least one other segment.